1. Field of the Invention
This invention relates to method of and apparatus for analyzing the composition of the surface of a specimen by electron spectroscopy and by secondary ion mass spectrometry. In particular, it provides an electron energy spectrometer combined with a secondary ion time-of-flight mass spectrometer, and a method by which a known electron energy spectrometer can be used as a secondary ion mass spectrometer.
2. Description of the Prior Art
Secondary ion mass spectroscopy (SIMS) and a variety of electron spectroscopies (e.g., X-ray photoelectron spectroscopy XPS, electron spectroscopy for chemical analysis ESCA, ultraviolet photoelectron spectroscopy UPS, etc) are commonly used for investigating the chemical structure of the surface of a solid specimen. In the former technique a primary beam of radiation impinges on the surface and releases secondary ions which are analyzed by a mass analyzer. In the latter techniques a primary beam of radiation releases from the surface of the specimen electrons whose energy is measured by a charged-particle energy analyzer to yield information about the chemical nature of the surface from which they were emitted.
Typical prior secondary ion mass spectrometers may comprise magnetic sector, quadrupole and time-of-flight mass analyzers, but time-of-flight analyzers are particularly attractive because of their ability to efficiently record a complete mass spectrum from a pulse of ions liberated from a surface.
Typical prior XPS and ESCA instruments comprise a cylindrical mirror analyzer or a part-spherical energy analyzer together with electrostatic or magnetic lenses to collect electrons emitted from the surface and transmit them to the analyzer. Descriptions of this general class of instrument may be found in U.S. Pat. No. 4,255,656 and in Hughes and Phillips, Surface and Interface Analysis, 1982 vol 4 (5) pp 220-226. Many such instruments are also capable of producing an energy-filtered image of the surface. In both SIMS and ESCA surface analysis apparatus, means for causing the emission of electrons and/or ions is also provided. This may comprise X-ray sources, electron sources, ion sources, UV and/or laser light sources. Both instruments also typically incorporate a UHV housing and vacuum system to prevent contamination of the surface under investigation.
In principle, a time-of-flight mass spectrometer for mass-analyzing ions generated by a pulsed ion source need comprise only an extraction or acceleration region, a field-free drift region and an ion detector. Assuming that the spatial separation of the ions in the bunch is minimal and their kinetic energies are equal at the point at which they enter the drift tube, the ions will separate in time according to their mass-to-charge ratios and arrive sequentially at the detector. In practice, however, the mass resolution is seriously reduced by a spread of the kinetic energies of ions in the bunch which results in ions of the same mass-to-charge ratio arriving at the detector at different times. It is known to minimize this problem by time-focusing, in which ions with higher velocities are made to travel a greater distance than slower ions of the same mass-to-charge ratio, thereby causing them to arrive at the detector at the same time.
The process was described in 1968 in U.S. Pat. No. 3,576,992 which teaches that the combination of a linear drift region with a curved drift region results in time focusing if correctly dimensioned. In this patent it is suggested that the curved drift region may comprise electrostatic ion-deflecting plates having a curved construction, typically cylindrical, spherical or toroidal. Many variations of the concept have been subsequently described e.g., Poschenrieder in GB Patent 1,405,180 (1975), Gohlke in U.S. Pat. No. 4,774,408 (1988), Bakker, Int. Journal Mass Spectrom and Ion Phys, 1971 vol 6 pp 291-5, and a review by Wollnik (Mass Spectrom. Rev. 1993, vol 12 pp 89-114).
Rose, Ondrey and Proch (Int. Journal Mass Spectrom. Ion Proc. 1992 vol 113 pp 81-98) teach that an electrostatic lens may be incorporated into a conventional time-of-flight mass spectrometer, resulting in an instrument capable of analyzing the energies of photoelectrons and determining the mass of ions by time-of-flight mass spectrometry. However, the ion/electron source in this instrument is a molecular beam, rather than a solid surface, and the lens comprises a grid and three electrodes disposed along the charged-particle axis. It does not therefore provide the curved drift region necessary to obtain effective time-focusing, as required by U.S. Pat. No. 3,576,992. To the best of the inventor's knowledge there are no other reports of instruments for both electron energy analysis and ion mass-spectroscopy in which the ions and electrons travel substantially along the same path.
Although there is a superficial similarity between the time-of-flight mass spectrometers of U.S. Pat. No. 3,576,992 and GB 1,405,180 and the conventional part-spherical electron energy analyzers frequently used for ESCA and XPS it will be understood that the charged-particle optical theory which determines their geometrical parameters is quite different. In the case of a time-of-flight spectrometer it is necessary to select the length of the linear portions of the flight path in relation to the length of the curved portion so that time-focusing is achieved. In contrast, in the case of an electron energy spectrometer these lengths are determined by the need for the analyzer system to focus electrons as well as disperse them according to their energies. Consequently the dimensions of apparatus suitable for time-of-flight mass spectroscopy will not in general be suitable for use of that apparatus as an electron spectrometer, and v.v. Thus, although an instrument comprising both an electron energy spectrometer for experiments such as XPS and ESCA and a time-of-flight secondary ion mass spectrometer is obviously a very versatile tool for surface analysis, the only reports of instruments incorporating both techniques teach separate mass and electron-energy analyzers fitted on a single vacuum housing, for example, Jahn, Petrat et al., J. Vac. Sci, Technol. 1994 vol A12 (3) p 671-676 and Siegbahn, J. Electron. Spectros. Related Phenom. 1990 vol 51 pp 11-36.